The treasurer shall, by notice published once not less than 30 days prior to the date designated in some financial journal having a national circulation, invite sealed proposals for the sale of any of the refunding bonds to the city. The notice shall state the amount available for the purchase of such bonds and shall specify the time and place when proposals will be opened. The treasurer shall also, at least 10 days prior to the date so designated, mail a copy of the invitation for sealed proposals to any bondholder who has theretofore in writing requested such notice.